The present invention relates generally to digital communications receivers. More specifically, but without limitation thereto, the present invention relates to adjusting the gain of a communications signal in selected frequency bands of a system frequency range.
The performance of analog-to-digital converters in digital communications receivers has progressed to the point where sampling speeds are sufficient to accommodate a communications signal carrying information on multiple carriers with a single analog-to-digital converter. After the multiple-carrier signal is sampled by the analog-to-digital converter, the digitized samples from the analog-to-digital converter are digitally demodulated to recover the information from each of the multiple carriers. The demodulator throughput may be increased by presenting the digitized samples from the analog-to-digital converter in a time-multiplexed format according to a Quality of Service (QoS) prioritization, or by implementing multiple demodulators to demodulate each of the multiple carriers concurrently. However, the advantages of a single analog-to-digital converter at the front end of a digital receiver have been offset by the difficulties encountered in the wideband multiple-carrier environment.
The dynamic range of an analog-to-digital converter is fixed by the number of bits of precision, while the dynamic range of a wideband multiple-carrier signal may vary with hardware configuration and environmental conditions, frequently exceeding the dynamic range of the analog-to-digital converter. Signals that exceed the dynamic range of the analog-to-digital converter result in clipping. For example, if an eight-bit analog-to-digital converter has an input voltage range of xe2x88x920.5 volts to +0.5 volts, then there are 256 digital samples equally distributed between xe2x88x920.5 volts and +0.5 volts that may be generated before the analog-to-digital converter exhibits clipping distortion. When clipping occurs, the analog-to-digital converter generates full-scale codes representing the full-scale input voltage for as long a time as the input signal voltage exceeds the full-scale input voltage of the analog-to-digital converter. Even though a desired signal may be well below the full-scale input voltage of the analog-to-digital converter, an interfering signal that exceeds the full-scale input voltage of the analog-to-digital converter could block the desired signal, even if the interfering signal is in a different frequency band, if both the desired signal and the interfering signal are in the passband presented to the analog-to-digital converter. The blocking of the desired signal results in partial or total service outages that may only be resolved by re-aligning the input signal levels below the full-scale input voltage of the analog-to-digital converter. The following are examples of some of the problems and limitations in a multi-carrier system:
1) The carriers may not all have the same signal level. The maximum signal level cannot exceed the full-scale input voltage of the analog-to-digital converter, and the minimum signal level must exceed the noise level of the analog-to-digital converter by a minimum signal-to-noise ratio to avoid losing or degrading the full bandwidth system performance.
2) The carriers may not all be of the same type. For example, QPSK, 16QAM, and 64QAM Data Over Cable Service Interface Specification (DOCSIS) carriers may all be present within the bandwidth presented to the analog-to-digital converter. Each of these carrier types has a separate minimum signal-to-noise requirement that must be maintained to preserve full bandwidth system performance.
3) Other services may also be present that are outside control of the Data Over Cable Service Interface Specification ranging protocol in the desired bandwidth, for example, video carriers may exist with the QPSK, 16QAM, and 64QAM DOCSIS carriers. The signals generated by the other services are independent of the DOCSIS ranging protocol and may result in a combined signal that exceeds the full-scale input of the analog-to-digital converter unless some type of filtering prior to analog-to-digital conversion is performed.
4) Spurious interference, including ingress signals, i.e., signals inserted into the distribution network by unsupported equipment, are outside of the overall link""s system control. Ingress signals may require the placement of permanent filtering at certain points of a system, which may not readily be performed or changed without field service calls.
5) Interference filters may be placed at the input of the receivers, but these filters are expensive and bulky, and plant specific ingress signals and service types present difficulties in optimizing such filters without some amount of trial and error. Also, readjustments may have to be performed as the system configuration changes over time.
6) Cable television plant levels are subject to variation resulting from changes in temperature due to weather and from changes in circuit components due to aging. Each plant therefore requires some amount of trial and error adjustment to align signal levels optimally.
7) The dynamic range of the analog-to-digital converter may be increased by adding bits of precision, however the cost of adding bits increases exponentially, and power consumption increases about four times for each bit of added precision.
The bandwidth of a received signal is typically limited by a superheterodyne receiver architecture, in which the intermediate frequency (IF) bandwidth is fixed. The superheterodyne architecture is reliable, however the fixed intermediate frequency bandwidth lacks flexibility in optimizing the bandwidth to the individual level requirements of a multi-carrier system. The superheterodyne receiver architecture may be extended to multiple switched intermediate frequency bandwidths to optimize the bandwidth presented to the analog-to-digital converter, however, this approach requires extensive circuitry for local oscillators, switch isolation, separate phase-locked loop (PLL) bandwidths for lock times, phase noise tradeoffs, as well as the intermediate frequency filters themselves. The increased printed circuit board size and cost of such receivers renders them impractical for competitive cost applications.
Direct down conversion receivers have a direct-conversion mixer that eliminates the extra intermediate frequency filters, local oscillators, and the IF down converter in conventional superheterodyne receivers. However, conventional direct down conversion receivers typically control the level of a composite multi-channel signal presented to an analog-to-digital converter by automatic gain control, disadvantageously reducing desired low-level signals below the signal level needed for demodulation to accommodate the limitations of the analog-to-digital converter imposed by other system signals that have a higher signal level and that lie in a frequency band outside that of the desired low-level signals.